U.S. patent application number 15/118172 was filed with the patent office on 2017-06-15 for improved electrostatic transducer.
The applicant listed for this patent is Warwick Audio Technologies Limited. Invention is credited to Brian ATKINS, Duncan BILLSON, Kevin WALSH.
Application Number | 20170171669 15/118172 |
Document ID | / |
Family ID | 50390813 |
Filed Date | 2017-06-15 |
United States Patent
Application |
20170171669 |
Kind Code |
A1 |
BILLSON; Duncan ; et
al. |
June 15, 2017 |
Improved Electrostatic Transducer
Abstract
An electrostatic transducer (100) comprises an electrically
conductive backplane member (102) having an array of through
apertures (112); a spacer member (104) disposed over the backplane
member (102), the spacer member (104) having an array of holes
(114) therethrough, the holes (114) each having a maximum lateral
dimension less than twice a minimum lateral dimension; and a
flexible electrically conductive membrane (106) disposed over the
spacer member (104). The transducer (100) is arranged in use to
apply an electrical potential which gives rise to an attractive
electrostatic force between the backplane member (102) and the
membrane (106) thereby moving portions of the membrane (106)
spanning said holes in the spacer member (104) towards said
backplane member (102).
Inventors: |
BILLSON; Duncan;
(Warwickshire, GB) ; ATKINS; Brian; (Osbaston,
GB) ; WALSH; Kevin; (Brynhyfryd, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Warwick Audio Technologies Limited |
Coventry, West Midlands |
|
GB |
|
|
Family ID: |
50390813 |
Appl. No.: |
15/118172 |
Filed: |
February 11, 2015 |
PCT Filed: |
February 11, 2015 |
PCT NO: |
PCT/GB2015/050375 |
371 Date: |
August 11, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04R 2201/401 20130101;
H04R 2201/403 20130101; H04R 19/005 20130101; H04R 19/02 20130101;
H04R 31/003 20130101 |
International
Class: |
H04R 19/02 20060101
H04R019/02; H04R 19/00 20060101 H04R019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2014 |
GB |
1402362.6 |
Claims
1. An electrostatic transducer comprising: an electrically
conductive backplane member having an array of through apertures; a
spacer member disposed over the backplane member, the spacer member
having an array of holes therethrough, the holes each having a
maximum lateral dimension less than twice a minimum lateral
dimension; and a flexible electrically conductive membrane disposed
over the spacer member; wherein the transducer is arranged in use
to apply an electrical potential which gives rise to an attractive
electrostatic force between the backplane member and the membrane
thereby moving portions of the membrane spanning said holes in the
spacer member towards said backplane member.
2. An electrostatic transducer comprising: an electrically
conductive backplane member having an array of through apertures; a
spacer member disposed over the backplane member, the spacer member
having an array of holes therethrough; and a flexible electrically
conductive membrane disposed over the spacer member; wherein the
transducer is arranged in use to apply an electrical potential
which gives rise only to an attractive electrostatic force between
the backplane member and the membrane thereby moving portions of
the membrane spanning said holes in the spacer member towards said
backplane member.
3. An electrostatic transducer as claimed in claim 2, wherein the
holes each have a maximum lateral dimension less than twice a
minimum lateral dimension.
4. An electrostatic transducer as claimed in claim 1 or 3, wherein
the ratio between the maximum and minimum lateral dimensions is
less than 1.5, preferably less than 1.2.
5. An electrostatic transducer as claimed in any preceding claim,
wherein the membrane is arranged so that it is not in contact with
the spacer member when zero electrical potential is applied.
6. An electrostatic transducer as claimed in claims 1 to 4, wherein
the membrane is held in contact with the spacer member.
7. An electrostatic transducer as claimed in claim 6, wherein the
membrane is held in contact with the spacer member by a mechanical
pre-tension, by bonding and/or by an electrical potential.
8. An electrostatic transducer as claimed in any preceding claim,
wherein the transducer is arranged in use to apply an electrical
potential which gives rise only to an attractive electrostatic
force between the backplane member and the membrane.
9. An electrostatic transducer as claimed in any preceding claim,
wherein the holes have a shape that is selected from the group
consisting of: circular, hexagonal, square and oval.
10. An electrostatic transducer as claimed in any preceding claim,
wherein the holes have a maximum lateral dimension between 1 mm and
50 mm, preferably between 10 mm and 40 mm, more preferably between
20 mm and 30 mm.
11. An electrostatic transducer as claimed in any preceding claim,
wherein the holes have a maximum lateral dimension between 2 and 50
times greater than the maximum lateral dimension of the apertures
in the backplane member, preferably between 10 and 40 times
greater, more preferably between 20 and 30 times greater.
12. An electrostatic transducer as claimed in any preceding claim,
wherein the spacing between the holes in the spacer member is
between 1 and 5 mm, preferably between 2 and 4 mm, more preferably
about 3 mm.
13. An electrostatic transducer as claimed in any preceding claim,
wherein every hole in the spacer member has the same size and
shape.
14. An electrostatic transducer as claimed in any of claims 1 to
12, wherein some holes in the array of holes have a different size
and/or a different shape from other holes in the array of
holes.
15. An electrostatic transducer as claimed in any preceding,
wherein the size, spacing, shape and/or pattern of the holes varies
across the surface of the spacer member.
16. An electrostatic transducer as claimed in any preceding claim,
wherein the holes are arranged in a hexagonal close packed
array.
17. An electrostatic transducer as claimed in any of claims 1 to
15, wherein the holes are arranged in a square lattice
arrangement.
18. An electrostatic transducer as claimed in any preceding claim,
wherein the holes have substantially tessellating shapes.
19. An electrostatic transducer as claimed in any preceding claim,
wherein the backplane, spacer and membrane are bonded together so
as to introduce a pre-tension to the membrane.
20. An electrostatic transducer as claimed in any preceding claim,
wherein the membrane is provided with a pre-tension such that when
the electrostatic potential reaches a maximum of its dynamic range,
the displacement of the portions of the membrane is less than or
substantially equal to the thickness of the spacer member.
21. An electrostatic transducer as claimed in any of claims 1 to
19, wherein the membrane is provided with a pre-tension to allow
contact between the membrane and the backplane during some or all
of the time that an electrical potential is applied.
22. An electrostatic transducer as claimed in any preceding claim,
wherein the thickness of the spacer member is between 15 .mu.m and
3 mm, preferably between 0.1 mm and 1 mm.
23. An electrostatic transducer as claimed in any preceding claim,
wherein bonding is provided between the backplane and spacer
members; between the spacer member and the membrane; or between
both the backplane and spacer members and the spacer member and the
membrane.
24. An electrostatic transducer as claimed in any preceding claim,
wherein the backplane, spacer member and membrane each comprise a
substantially planar sheet.
25. An electrostatic transducer as claimed in any preceding claim,
wherein the backplane member is a composite layer comprising a
polymer sheet having a conductive layer applied thereon by
metallization.
26. An electrostatic transducer as claimed in any preceding claim,
wherein backplane member a thickness between 0.2 mm and 5 mm.
27. An electrostatic transducer as claimed in any preceding claim,
wherein the apertures have a maximum lateral dimension between 0.5
mm and 2 mm.
28. An electrostatic transducer as claimed in any preceding claim,
wherein the spacing between the apertures is between 0.5 mm and 5
mm.
29. An electrostatic transducer as claimed in any preceding claim,
wherein the spacer member is made from a polymer.
30. An electrostatic transducer as claimed in any preceding claim,
wherein the spacer member comprises a conductive layer overlaid on
an insulating substrate.
31. An electrostatic transducer as claimed in any preceding claim,
wherein the flexible electrically conductive membrane comprises an
electrically conductive layer overlaid onto an electrically
insulating layer.
32. An electrostatic transducer as claimed in any preceding claim,
wherein the membrane is between 4 .mu.m and 0.5 mm thick,
preferably between 6 .mu.m and 0.1 mm thick.
33. An electrostatic transducer as claimed in any preceding claim,
the thickness of each member varies across the transducer.
Description
[0001] This invention relates to an electrostatic transducer and is
particularly but not exclusively concerned with a loudspeaker
suitable for reproducing audio signals.
[0002] A traditional electrostatic loudspeaker comprises a
conductive membrane disposed between two perforated conductive
backplates to form a capacitor. A DC bias is applied to the
membrane and an AC signal voltage is applied to the two backplates.
Voltages of hundreds or even thousands of volts may be required.
The signals cause an electrostatic force to be exerted on the
charged membrane, which moves to drive the air on either side of
it.
[0003] In U.S. Pat. No. 7,095,864, there is disclosed an
electrostatic loudspeaker comprising a multilayer panel. An
electrically insulating layer is sandwiched between two
electrically conducting outer layers. The insulating layer has
circular pits on one of its sides. It is said that when a DC bias
is applied across the two conducting layers, portions of one of the
layers are drawn onto the insulating layer to form small drum skins
across the pits. When an AC signal is applied, the drum skins
resonate, and parts of that conducting layer vibrate to produce the
required sound.
[0004] In WO 2007/077438 there is disclosed an further type of
electrostatic loudspeaker comprising a multilayer panel. An
electrically insulating layer is sandwiched between two
electrically conducting outer layers. In this arrangement, one of
the outer conducting layers is perforated and, for example, may be
a woven wire mesh providing apertures with a size of typically 0.11
mm.
[0005] In US 2009/0304212 there is disclosed an electrostatic
loudspeaker comprising a conductive backplate provided with an
array of vent holes and an array of spacers. Over this is
positioned a membrane comprising a dielectric and a conductive
film. The space between the backplate and the membrane is about 0.1
mm and it is said that a low voltage supplied to the conductive
backplate and the conductive film will push the membrane to produce
audio.
[0006] One problem with electrostatic loudspeakers of this type is
obtaining sufficient displacement of the membrane. WO 2012/156753
discloses an electrostatic transducer comprising an electrically
conductive first layer having through apertures, a flexible
insulating second layer over the first layer, and a flexible
electrically conductive third layer disposed over the second layer.
Spaces are provided between the first and second layers or between
the second and third layers. Spaces between the first and second
layers allows greater freedom of movement of the second and third
layers, allowing greater displacement of the second and third
layers. Spaces between the second and third layers were also found
to improve acoustic performance.
[0007] However, there remains a need for further improvement in the
acoustic performance of electrostatic transducers of this type.
[0008] When viewed from a first aspect the invention provides an
electrostatic transducer comprising: [0009] an electrically
conductive backplane member having an array of through apertures;
[0010] a spacer member disposed over the backplane member, the
spacer member having an array of holes therethrough, the holes each
having a maximum lateral dimension less than twice a minimum
lateral dimension; and [0011] a flexible electrically conductive
membrane disposed over the spacer member; [0012] wherein the
transducer is arranged in use to apply an electrical potential
which gives rise to an attractive electrostatic force between the
backplane member and the membrane thereby moving portions of the
membrane spanning said holes in the spacer member towards said
backplane member.
[0013] Thus it will be seen by those skilled in the art that the
holes provided in the spacer member cooperate with the membrane to
provide an array of regions where a `drum-skin` effect is produced.
Optimal performance has been found to be achieved when the holes
have similar dimension all the way round. The ratio between the
maximum and minimum lateral dimensions may be less than 1.5 e.g.
less than 1.2.
[0014] Furthermore the tension generated in the membrane when
portions are moved towards the backplane member provides a return
force when there is a decrease in the electrostatic potential (and
so reduction in the electrostatic force). The present invention
therefore improves on previous, similar transducers by effectively
introducing a "return spring" into the transducer, significantly
improving its acoustic performance. For example such arrangements
may increase the usable frequency range and improve the overall
quality of the sound generated by a transducer. This is illustrated
by a 6 dB increase in the sound pressure level between 200 Hz and 5
kHz having been observed in some embodiments.
[0015] The membrane may be arranged so that it is not initially in
contact with the spacer member--i.e. when zero electrical potential
is applied. In such cases, the membrane may be brought into contact
with the spacer by the application of the electrical potential,
which attracts the membrane to the backplane member. The portions
of the membrane spanning the holes in the spacer member are thus
able move in response to the electrical potential in the manner
described above. Equally the membrane may be held in contact with
the spacer member e.g. by a mechanical pre-tension, by bonding or
by an electrical potential. For example, a d.c. bias potential may
be applied to maintain the membrane in contact with the spacer,
while an a.c. drive signal is applied in addition to the d.c.
signal to drive the motion of the portions spanning the holes.
[0016] The invention as outlined above could be applied to
so-called push-pull transducers in which two backplane members are
provided on either side of the membrane to move it in both
directions. However in preferred embodiments the transducer is
arranged in use to apply an electrical potential which gives rise
only to an attractive electrostatic force between the backplane
member and the membrane. In such an arrangement only a single
backplane member is necessary. The return force mentioned
hereinabove allows good acoustic performance to be achieved
nonetheless.
[0017] Such an arrangement is novel and inventive in its own right,
therefore when viewed from a second aspect the invention provides
an electrostatic transducer comprising: [0018] an electrically
conductive backplane member having an array of through apertures;
[0019] a spacer member disposed over the backplane member, the
spacer member having an array of holes therethrough; and [0020] a
flexible electrically conductive membrane disposed over the spacer
member; [0021] wherein the transducer is arranged in use to apply
an electrical potential which gives rise only to an attractive
electrostatic force between the backplane member and the membrane
thereby moving portions of the membrane spanning said holes in the
spacer member towards said backplane member.
[0022] Any suitable shape for the holes may be used, but in
preferred embodiments the holes each have a maximum lateral
dimension less than twice a minimum lateral dimension for the
reasons given above.
[0023] Except where explicitly provided otherwise, the features
discussed hereinbelow may be applied either to the first aspect of
the invention or to the second aspect of the invention.
[0024] The size, shape, spacing and pattern of the holes in the
spacer member may affect the magnitude of the tension introduced to
the membrane, as well as affecting the regions of the membrane
where tension is created. Accordingly, the size, shape, spacing and
pattern of the hole may be optimised to generate a desired amount
of tension, or to maximise the tension generated in the membrane.
In some embodiments the holes have a shape that is selected from
the group consisting of: circular, hexagonal, square and oval.
However, other shapes are possible.
[0025] The holes in the spacer member may be any suitable size,
however in some embodiments the holes have a maximum lateral
dimension between 1 mm and 50 mm, e.g. between 10 mm and 40 mm,
e.g. between 20 mm and 30 mm, e.g. about 25 mm. In some embodiments
the holes in the spacer member are larger than the apertures in the
backplane member. The holes may have a maximum lateral dimension
between 2 and 50 times greater than the maximum lateral dimension
of the apertures in the backplane member, e.g. between 10 and 40
times greater, e.g. between 20 and 30 times greater, e.g. around 25
times greater.
[0026] The spacing between the holes in the spacer member may have
any suitable dimension. However, as sound may be generated by the
membrane only or mainly where it is free to vibrate over the holes
of the spacer member, it is preferable that the spacing between the
holes is much less than the size of the holes. However, the spacing
should not be so small as to adversely affect the support provided
to the membrane by the spacer member, or so small that damage is
caused to the membrane due to the pressure of the reaction force of
the spacer member. Accordingly, in preferred embodiments the
spacing between the holes in the spacer member is between 1 and 5
mm, e.g. between 2 and 4 mm, e.g. about 3 mm.
[0027] In some embodiments, every hole in the spacer member has the
same size and shape. However, this is not essential: it is possible
for holes in the spacer member to have different sizes and
different shapes. For example, the spacer member could have an
array of holes comprising some holes that are 20 mm and circular
and some holes that are 30 mm and circular. As another example, the
spacer member could have some holes that are hexagonal, and some
holes that are square. The size, spacing, shape and/or pattern of
the holes may vary across the surface of the spacer member. For
example, larger holes may be provided towards the centre of the
spacer member and smaller holes towards the edge. As another
example, the spacer member could be provided with a hexagonal array
of hexagonal holes in one portion of the spacer member and a square
array of square holes in another portion of the spacer member.
[0028] The holes may be arranged in any suitable pattern or
arrangement. However, as discussed above, it is preferable in some
circumstances that the spacing between the holes is not too large
so as to maximise the area of the membrane that can vibrate over
the holes of the spacer member. Therefore, in some embodiments, the
holes are arranged in a hexagonal close packed array. In some other
embodiments the holes are arranged in a square lattice arrangement.
The holes may be provided with a suitable shape to minimise the
spacing between the holes, i.e. substantially tessellating shapes.
For example, if the array is a hexagonal close packed array, the
holes may have a hexagonal shape (i.e. a honeycomb arrangement). If
the holes are arranged in a square lattice arrangement, the holes
may have a square shape. However, this is not necessarily the case.
For example the holes could be circles arranged in a square lattice
arrangement or in a hexagonal close packed arrangement. Other
lattice arrangements are possible, and in some embodiments the
holes are arranged randomly.
[0029] As there may be advantages associated with the
aforementioned tension in the membrane, it is desirable to optimise
the structure of the transducer so as to optimise the tension in
the membrane. A factor that may affect the performance of the
transducer in this way is any tension of the membrane that is
introduced at the manufacturing stage of the transducer. For
example when the backplane, spacer and membrane are assembled, they
may be bonded together (e.g. at the edges of the members, or across
the surface of the members, as discussed further herein below) so
as to introduce a pre-tension to the membrane.
[0030] It may be particularly desirable to maximise the magnitude
of vibrations of the membrane, as this may maximise the acoustic
response to the applied electrostatic potential. However, should
the membrane be displaced too far, it may contact the backplane
member. The presence of the spacer member prevents the membrane
contacting the backplane member across the entire surface of the
membrane, and the transducer will still function if the membrane
touches the backplane member in a small region corresponding to the
centre of the holes in the spacer member.
[0031] In some embodiments there is no contact between the membrane
and the backplane member. Thus in some embodiments the membrane is
provided with a pre-tension when the transducer is manufactured,
such that when the electrostatic potential reaches a maximum of its
dynamic range, the displacement of the portions of the membrane is
less than or substantially equal to the thickness of the spacer
member.
[0032] Conversely, in some embodiments the membrane does touch the
backplane. The membrane may be provided with a pre-tension to allow
contact between the membrane and the backplane during some or all
of the time that an electrical potential is applied. For example,
the membrane may touch the backplane only when the electrical
potential is high. Alternatively, the membrane may remain in
contact with the backplane while the electrical potential is
applied, and move in response to variation in the electrical
potential, so that the area in contact with the backplane varies as
the membrane moves.
[0033] It will be appreciated from the above that the desired
pre-tension of the membrane may depend to some extent of the
thickness of the spacer member. The spacer member can have any
suitable thickness, however the thickness of the spacer member may
be between 15 .mu.m and 3 mm, e.g. between 0.1 mm and 1 mm, e.g.
about 0.5 mm. As discussed above, the backplane, spacer and
membrane may be bonded at their edges. Additionally or
alternatively, these members may be bonded together, either in part
or across their entire surfaces. For example, the members may be
bonded at bonding lines spaced across them. As another example, the
membrane may be bonded to the spacer member at multiple discrete
points between some of the holes in the spacer member. There may be
bonding provided between the backplane and spacer members, between
the spacer member and the membrane, or between both the backplane
and spacer members and the spacer member and the membrane. The
bonds between the members may have negligible thickness or may
serve as further spacers separating the members.
[0034] The backplane, spacer and membrane may each comprise a
substantially planar sheet.
[0035] The electrically conductive backplane member may be made of
any suitable material or combination of materials. The electrically
conductive backplane member may be rigid, but may be semi-rigid or
flexible. For example, the backplane member may be a composite
layer comprising a polymer sheet having a conductive layer applied
thereon by metallization, e.g. by vapour deposition. The conductive
layer may comprise aluminium. Alternatively, the backplane member
may comprise a metal sheet. In some embodiments, the metal sheet is
aluminium. The backplane member may have any suitable thickness,
e.g. between 0.2 mm and 5 mm, e.g. about 1 mm.
[0036] The apertures in the backplane member may be circular. The
apertures may have a maximum lateral dimension (parallel to the
median plane of the backplane member) of between 0.5 mm and 2 mm,
e.g. about 1 mm. The spacing between the apertures may be between
0.5 mm and 5 mm, e.g. about 1 mm. The term "spacing" as used herein
with reference to aperture spacing has the meaning of the distance
between the closest edges of adjacent apertures (i.e. the thickness
of the material between the apertures), rather than, for example,
the distance between the centres of adjacent apertures.
[0037] The spacer member may be made of any suitable material or
combination of materials, but preferably it is made from a polymer,
e.g. Mylar. The spacer member may be rigid, semi-rigid or
flexible.
[0038] In some embodiments the spacer member is electrically
insulating. However the Applicant also envisages that the spacer
member could be conductive--e.g. by having a conductive layer
overlaid on an insulating substrate to which the electrical
potential is applied, such that the membrane is also attracted to
the conductive layer of the spacer member. This may provide an
advantage that a greater attractive force is provided (due to the
greater proximity of the membrane to the conductive layer on the
spacer member compared with its proximity to the backplane
membrane). A smaller potential may therefore be needed to bring the
membrane into contact with the spacer member. The conductive layer
may extend over the walls of the apertures. This may provide an
advantage that the attraction of the membrane to the conductive
layer may contribute to the movement of the membrane portions
spanning the holes.
[0039] The flexible electrically conductive membrane may be made of
any suitable material or combination of materials. It may be made
entirely from electrically conductive material or it may be made
only partially of electrically conductive materials, e.g. it may
comprise an electrically conductive layer overlaid onto an
electrically insulating layer. Preferably it is made from a
metallised polymer sheet. For example, the membrane may be made
from a Mylar polymer sheet having a layer of aluminium deposited
thereon by metallization. The membrane may be between 4 .mu.m and
0.5 mm thick, e.g. 6 .mu.m and 0.1 mm thick, e.g. about 10 .mu.m
thick.
[0040] The thickness of each member may be constant, or may vary
across the transducer.
[0041] The holes may each have a maximum lateral dimension less
than twice a minimum lateral dimension. The backplane member may be
electrically conductive. The spacer member may be electrically
insulating. Preferably the transducer is arranged in use to apply
an electrical potential which gives rise only to an attractive
electrostatic force between the conductive layer and the
membrane.
[0042] Certain preferred embodiments of the present invention will
now be described, by way of example only, with reference to the
accompanying drawings in which:
[0043] FIG. 1 is a diagrammatic section through a transducer in
accordance with one embodiment of the invention, showing the
position of a flexible electrically conducting membrane disposed
over a spacer member having holes therethrough, when zero
electrical potential is applied to the transducer;
[0044] FIG. 2 is a plan view of the spacer member of the transducer
of FIG. 1, showing the holes through the spacer member;
[0045] FIG. 3 is a diagrammatic section through the transducer of
FIG. 1, showing the position of the membrane when a non-zero
electric potential is applied to the transducer;
[0046] FIG. 4 is a diagrammatic section through a transducer in
accordance with another embodiment of the invention, wherein a
conductive layer is overlaid on the spacer member;
[0047] FIG. 5 is a diagrammatic section through the transducer of
FIG. 4, showing the position of the membrane when a non-zero
electric potential is applied to the transducer.
[0048] FIG. 1 shows a transducer 100 comprising a backplane member
102, with a thickness of 1 mm. The backplane member 102 is made
from an aluminium sheet, although other materials or combinations
of materials could be used. Disposed over the backplane member is
an insulating spacer member 104. The spacer member 104 is 0.3 mm
thick, and is made from the polymer Mylar.
[0049] Disposed over the spacer member 104 is a composite membrane
106. The membrane 106 comprises a polymer sheet of 10 .mu.m
thickness, with an aluminium layer 110 deposited thereon via
metallisation. In the present embodiment, the aluminium layer is
provided on the surface of the polymer sheet 108 that faces away
from the spacer member 104. However, in other embodiments, the
membrane may comprise a conducting layer on the side of the polymer
layer facing the spacer member, or a conducting layer could be
sandwiched between two polymers sheets. In some embodiments,
instead of the composite membrane there could be a single flexible
conducting layer.
[0050] The backplane member 102 is provided with an array of
through apertures 112. The apertures 112 are circular with a
diameter of 3 mm, and with an inter-aperture spacing of 2 mm. The
through apertures 112 are positioned in a regular square lattice
arrangement.
[0051] The spacer member 104 is provided with an array of through
holes 114. As shown in FIG. 2, the through holes 114 have a
hexagonal shape and are arranged in a hexagonal close packed
arrangement, i.e. in a honeycomb arrangement. They have a maximum
lateral dimension (vertex to vertex, as indicated by arrows A) of
22 mm and a minimum lateral dimension (edge to edge) of 19 mm. The
spacing between the holes 114 defines an inter-hole wall 116. The
inter-hole wall 116 has a thickness (as indicated by arrows B) of 3
mm.
[0052] In use, a varying electrostatic potential is applied to the
backplane member 102, and the conducting aluminium layer 110 of the
membrane 106. This is shown in FIG. 3. The electrical potential
consists of a DC potential (250V) added to an AC drive signal
(+1-200V), the latter corresponding to the desired sound. This
results in a potential that can vary between 50V and 450V,
depending on the desired sound waveform. The electrical potential
causes an attractive electrostatic force between the backplane
member 102 and the membrane 106 that depends on the strength of the
potential. The membrane 106 has portions 118 that are displaced
towards the backplane member 102 as a result of the force, moving
the air around them. An acoustic response to the electrical signal
is thereby produced.
[0053] As the portions 118 deform in order to move closer to the
backplane member 102, tension is created in the portions 118 of the
membrane spanning the hole 114. This tension provides a biasing
force biasing the portions 118 back towards their equilibrium
positions so that when the electrical potential is decreased, the
biasing force due to tension provides a return spring effect,
restoring the portions 118 of the membrane 106 towards their
equilibrium positions, thereby improving the acoustic performance
of the transducer.
[0054] In the present embodiment, no bonding is provided between
the members 102, 104, 106. However, in other embodiments the
members 102, 104, 106 could be bonded together in part or across
their entire surface where they are in contact. For example, the
membrane 106 could be bonded in some places where it contacts the
upper surface of the inter-hole walls 116. Similarly, the backplane
member 102 could be bonded to the spacer member 104 in some or all
places where it contacts the bottom of the inter-hole walls
116.
[0055] FIG. 4 shows a transducer 400 having corresponding features
to those of the embodiment of FIG. 1, i.e. a backplane member 402;
a spacer member 404 disposed over the backplane member 402; and a
composite membrane 406. In addition, in this embodiment however a
conductive metal layer 420 is applied over the spacer member 404.
In this embodiment the metal layer 420 is in fact continued over
the backplane member 402 in which case it is not necessary for the
backplane member to be conducting. The substrate of the spacer
member 404 is 0.3 mm thick, and is made from the polymer Mylar. The
conductive layer 420 is created by metallization of the spacer
member 404 and the backplane member 402, so that the conductive
layer 420 covers the exposed upper surfaces of the spacer member
404 and the backplane member 402, as well as the walls of the holes
in the spacer member 404. The conductive layer also extends
partially down the walls of the apertures in the backplane member
402. In other embodiments separate metal layers could be applied to
the spacer member and the backplane member or a metal layer could
be applied to the spacer member only. The membrane 406 comprises a
polymer sheet of 10 .mu.m thickness, with an aluminium layer 110
deposited thereon via metallisation.
[0056] In use, a varying electrostatic potential is applied to the
conductive layer 420, and the conducting aluminium layer 410 of the
membrane 406. This is shown in FIG. 5. The electrical potential
consists of a DC potential (250V) added to an AC drive signal
(+/-200V), the latter corresponding to the desired sound. This
results in a potential that can vary between 50V and 450V,
depending on the desired sound waveform. The electrical potential
causes an attractive electrostatic force between the conductive
layer 420 and the membrane 406 that depends on the strength of the
potential. The membrane 406 has portions 418 that are displaced
towards the conductive layer 420, and thus towards the backplane
member 402, as a result of the force, moving the air around them.
An acoustic response to the electrical signal is thereby
produced.
[0057] The portions 418 deform in order to move closer to the
conductive layer 420 (and thus to the backplane member 402),
creating tension in the portions 418 of the membrane spanning the
hole 414. As in the previous embodiment, this tension provides a
biasing force biasing the portions 418 back towards their
equilibrium positions so that when the electrical potential is
decreased, the biasing force due to tension provides a return
spring effect, restoring the portions 418 of the membrane 406
towards their equilibrium positions, thereby improving the acoustic
performance of the transducer.
[0058] It will be appreciated by those skilled in the art that only
two possible embodiments have been described and that many
variations and modifications are possible within the scope of the
invention. For example, each of the members may have a different
thickness, or may be made from alternative materials. The holes
could have a different shape, size, spacing or pattern, and the
apertures may have different shape, size, spacing or pattern.
* * * * *